Inflammatory bowel disease (IBD) affects approximately 1.4 million individuals in the US and several million worldwide. One of the major discoveries in the last 20 years in IBD research has been the key role cytokines play in intestinal inflammation and the development of anti-cytokine treatments for IBD patients, including anti-TNF1 therapies. In spite of these important advances, much more needs to be learned concerning the basic mechanism(s) by which cytokines regulate normal gut homeostasis and mediate chronic intestinal inflammation and IBD. The present competitive renewal application will focus on two TNF superfamily members and their receptors, TL1A/DR3 and TWEAK/Fn14, for which increasing evidence suggests a major role in the pathogenesis of human and experimental CD. The central hypothesis of this proposal is that TL1A/DR3 and TWEAK/Fn14 play a protective role in normal gut homeostasis and in early disease compared to a proinflammatory role in the chronic phase of ileitis. The following three aims are designed to continue our ongoing efforts to understand the role of cytokines in mediating intestinal inflammation, with a particular focus on TL1A and TWEAK. 1) Determine the role of TL1A/DR3 in intestinal inflammation. We will first study the effect of intestinal injury induced by DSS in mice deficient in TL1A and DR3. In addition, we will test the effect of monoclonal neutralizing antibodies against TL1A in the early versus late phases of CD-like ileitis in SAMP and TNF?ARE mice. Finally, we will cross TL1A-/- and DR3-/- mice with TNF?ARE and SAMP mice. 2) Determine the role of TWEAK/Fn14 in intestinal inflammation. Recent studies in human and experimental IBD suggest that TWEAK/Fn14 may play an important in the pathogenesis of intestinal inflammation. We will take advantage of state-of-the-art genetically manipulated murine models to determine the role of TWEAK/Fn14 in regulating acute intestinal injury induced by DSS. In addition, using a series of cross-breeding experiments with our SAMP and TNF?ARE mice, we will define the role of TWEAK/Fn14 in early versus late phases of chronic ileitis. Finally, by using adoptive transfer models of T cell- mediated colitis and bone marrow chimera models, we will determine the role of hematopoietic and epithelial- derived TWEAK in mediating CD-like ileitis. 3) Define the function of TL1A/DR3 and TWEAK/Fn14 in intestinal mucosal cells. We will focus our attention on performing a variety of in vitro studies using intestinal mucosal cells isolated from control and SAMP or TNF?ARE mice. We will study the ability of recombinant TL1A and TWEAK to stimulate Th1, Th2, and Th17 responses in mesenteric and lamina propria mononuclear cells of SAMP and TNF?ARE mice. In addition, we will study the effects of TL1A and TWEAK on primary isolated intestinal epithelial cells and intestinal barrier function. The overall objective of this proposal is to investigate novel cytokine-based therapeutic strategies aimed at treating patients with IBD and begin to develop a cure for this devastating disease.